1. Field of Invention
The invention relates to double walled underground storage containers or tanks for fluids such as heating oil which endanger the ground water, having an inner and outer wall which preferably comprise a thermosetting plastics material, advantageously glass fiber reinforced plastic, and a leak monitoring space situated between these two which is filled with a continuous air and liquid permeable material bracing the inner wall against the outer wall.
2. Description of Prior Art
In a known heating oil container of this type (German Gebrauchsmuster No. 1 985 588) the leak monitoring space comprises an intermediate layer of porous open-pored foamed material. By producing the double wall with an intermediate layer in a single process one achieves a unitary container body which, however, may not be heavily loaded and which may not be termed a static, or in any way substantial, load bearing unit because the forces acting on the outer or inner walls cause a yielding and compression of the foamed layer so that, on the one hand, the inner and outer walls undergo a different deformation and must be correspondingly dimensioned, whilst on the other hand the permeability of the leak monitoring space and thus its whole function is impaired.
Double walled containers are known (German Patent Specification No. 1 272 830) for the purpose of achieving a satisfactory resistance against forces acting on the storage container from the outside in which the outer wall comprises steel and the inner wall comprises glass fibre reinforced polyester resin. In order to fix the geometrical shape of the inner wall and to achieve a uniform intermediate space the space is filled with a strengthening filling material in the form of a granular, preferably spherical, material provided with an adhesive so as to maintain the porosity of the intermediate space. In such containers substantially all those forces acting from the outside are taken by the outer wall which must be correspondingly strongly dimensioned, whilst when the dimensions are not correct the filling material braces the inner wall when in the empty state, but does not prevent a change of position of the latter upon loading.
Furthermore it is known in connection with heating oil containers with a double glass fiber reinforced plastic wall (German Offenlegungsschrift No. 1 939 150), to fill the intermediate space between the inner and outer walls with impermeable cast concrete or rigid polyurethane foam. Such containers exhibit good static properties, however the required wall strengths are too great. Above all, however, they cannot be employed in those areas in which leak monitoring is necessary as a consequence of particular ground water protection installations. However, because of their three-layered construction which permits of no leak control between the inner and outer walls such containers are classed by the authorities with the single walled containers.
Finally, a storage container of synthetic resin concrete in known (French Patent Specification No. 2 101 515), the inside of which is provided with a layer of synthetic resin so as to achieve the required seal and on its outside is given a coating of glass fiber reinforced plastic so as to achieve adequate mechanical strength. The synthetic resin concrete layer is not fluid tight so the above mentioned inner seal has to be provided. However, this plastic concrete layer may neither be regarded nor is it suitable as a leak monitoring space in any way since the mentioned permeability is merely such as also occurs in normal concrete. For these known containers expanded clay is suggested as the filling material which, as desired, leads to a considerable reduction in weight of the whole container. However, the expanded clay results in a closed pore structure and not a continuous leak monitoring space. On the other hand when using expanded clay one cannot do without completely filling the space remaining within the filling material structure because otherwise the strength would be insufficient for a container which is to be used underground.
3. Object of Invention
The main object of the invention to produce a storage container having two plastics walls taking up tension, compression and transverse forces which at the same time enables leaks to be monitored whilst avoiding the disadvantages of the known storage containers.